When two objects of dissimilar materials are rubbed together electrons are transferred from one material to the other through the process of triboelectrification. The objects become statically charged with one material accumulating positive charge and the other material accumulating negative charge. The process of safely discharging electrostatic charges or preventing or minimizing their occurrence, e.g. in a manufacturing or workplace environment, is accomplished through ESD devices. These devices have the effect of reducing static electricity charges on a person's body or equipment, for example to prevent fires and explosions when working with flammable liquids and gases, or to prevent damage to static-sensitive objects such as electronic components or devices.
The term Electrostatic discharge (ESD) device as used here includes conductive and dissipative devices, films and adhesives There are many standards relating to ESD. The ESD Association (www.esda.org) has published 35 standards covering ESD in the electronics environment. CENELEC has issued a European electrostatic standard EN100015—Protection of Electrostatic Sensitive Devices.
ESD devices have many areas of use, such as:                ESD packaging devices including films, bags, and rigid structures used to contain devices, such as graphics cards or hard disk drives under transport or storage. Such films can also be used in the production of batteries or capacitors, forming a conductive barrier inside the battery or capacitor.        ESD garment devices such as clothes and shoes, used in many workplaces.        ESD agents or compounds used for treatment of materials or their surfaces in order to reduce or eliminate build-up of static electricity        ESD mats and floors, ranging from small mats for keyboards and mice and larger mats or entire floors        ESD workstations and work surfaces provide an electrical path to ground for the controlled dissipation of any static potential on materials that contact the surface.        ESD parts such as gaskets.        
The terms conductive and dissipative can broadly be defined as:                Conductive: Materials with a resistance of between 1 kΩ and 1 MΩ        Dissipative: Materials with a resistance of between 1 MΩ and 1 TΩ        
The Electrostatic Discharge Association's document ESD ADV1.0-2009 provides the following definitions applicable in the current context:                conductive material, resistivity: A material that has a surface resistivity less than 1×10E5 ohms/square or a volume resistivity less than 1×10E4 ohm-cm.        conductive material, resistance: A material that has a surface resistance of less than 1×10E4 ohms or a volume resistance of less than 1×10E4 ohms.        conductive flooring material: A floor material that has a resistance to ground of less than 1.0×10E6 ohms.        dissipative floor material: Floor material that has a resistance to ground between 1.0×10E6 and 1.0×10E9 ohms.        dissipative materials: A material that has a surface resistance greater than or equal to 1×10E4 ohms but less than 1×10E11 ohms or a volume resistance greater than or equal to 1×10E4 ohms but less than 1×10 μl ohms.        
The present invention concerns conductive and dissipative ESD devices, and we use the common term ESD device for these. The term “antistatic” is also a common synonym for “electrostatic discharge”, e.g. used in “Antistatic device” as a synonym for ESD device; i.e. we use the term ESD device as equivalent to antistatic device.
In the production of such conductive and dissipative devices like antistatic films and bags, shoes, mats and floors, polymers are well suited for use at the outer surface. Other materials that similarly can change viscosity during production are also suited. The materials are manufactured into a film or a sheet which can be laminated or a liquid coating which can be sprayed on or the device dipped in, and then cured by spontaneous reaction of the material, or accelerated by the use of e.g. heat or UV light
Electrically conductive polymer containing materials can be based on the mixture of a polymer matrix and conductive particles (fillers) embedded into this matrix, or inherently conductive polymers.
Electrically conductive or dissipative polymer materials which are of interest to this invention are based on the mixture of a nonconductive polymer matrix and conductive particles (fillers) embedded into this matrix; Inherent conductive polymers are also known and alloyed with nonconductive polymers can form conductive or dissipative materials.
The addition of filler changes some of the bulk properties of the polymer matrix. These changes are frequently undesirable, e.g. a decrease in material strength and transparency and change in colour. It is important to lower the filler content to minimize these effects.
In the present invention the polymer matrix can be an adhesive and the electrically conductive particles; metal, metal oxides, metal-colloid particles, or carbon particles such as carbon nanotubes (CNTs). The materials can also be directionally conductive.
The electrically conductive or dissipative polymer films are usually produced by mixing the filler material with a polymer resin and in order to have a conductive mixture the amount of filler material must exceed the percolation threshold. Mixed systems have limited lifetime and must be re-mixed prior to use. A common problem is that a film or layer even when manufactured to a uniform thickness, will have non-uniform conductivity because the filler material will not distribute evenly. The problem is well-known in polymer physics and stems from the mutual incompatibilities of filler materials and polymer matrix, which means that only small amount of fillers can be mixed with the matrix to lead to the stable mixture. Higher amounts will macrophase separate with time. Therefore, this problem is fundamental in nature. Moreover, the mixing process applied for higher filler quantities must be so vigorous that the filler particles may get broken.
U.S. Pat. No. 4,269,881 and U.S. Pat. No. 5,348,784 teaches the production of carpet products where conductive fibres are mixed into the base of an adhesive. U.S. Pat. No. 4,724,187 teaches the similar for conductive laminate flooring.
US2005/0206028A1 teaches electrically conductive flooring formed of a conductive loaded resin-based material that comprises micron sized conductive powder, conductive fibre or a combination in 20% to 50% by weight of the total conductive resin. WO2010018094A2 teaches a similar invention for a substrate-free conductive surface.
U.S. Pat. No. 4,101,689 teaches an electrically conductive floor covering comprising a sheet of a thermoplastic synthetic resin, substantially non conducting, said sheet having a plurality of holes penetrating there through and electrically conductive material filling said holes.
U.S. Pat. No. 4,944,998 teaches a surface covering vinyl floor tile product having static dissipative electrical properties and a method of producing the same.
U.S. Pat. No. 7,060,241B2 discloses an electrically conductive film using single-walled CNT giving conductivity and transparency. The CNT can be oriented by exposing the films to a shearing step.
In order to increase signal transmission capability or dissipative discharge without having to increase the amount of conductive filler material, conductive films can be made anisotropic.
Anisotropic films can also be designed so that they have insulating properties in certain directions.
In EP 1809716 is described a method for making a directionally conductive adhesive based on CNTs. A tape having an insulation base and a parallel arrangement of CNTs acting as electrical contact points is made by growing carbon CNTs on a material used in the tape or arranging CNTs on the tape before adding the adhesive part to the tape.
In U.S. Pat. No. 5,429,701 is described how electric interconnection between discreet individual conductors of soft magnetic metal in two layers is achieved by adjoining the conductors by a conductive adhesive. The adhesive have particles of soft magnetic metal and by applying a magnetic field the particles can be gathered in an area between the conductors.
It is known that dipolar rigid asymmetric particles or molecules can be aligned by an electric field; this is especially used for small molecular weight liquid crystals.
In these cases the material having permanent dipole moments is fluid under normal conditions, a fact which makes electric field alignment possible.
Aligned structures of infusible conductive carbon particles, like CNTs, are known to be formed by chemical vapour deposition or spinning.
A method for the directional growth of CNTs is shown in U.S. Pat. No. 6,837,928. CNTs are grown in an electric field which directs their growth and thus leads to aligned CNTs when the growing procedure is completed.
Mixing of CNCs with diverse materials has been described in document WO2006052142. In this description CNCs form isotropic mixture with the matrix.
In document WO2008009779 electric field is used to induce sintering in nanoparticle coating.
Electric field alignment of carbon nanocone (CNC) material has been demonstrated in Sv{dot over (a)}sand et al. Colloids & Surf A Physicochem. Eng. Aspects 2007 308, 67 and 2009 339 211. In these articles it, is shown that nanocone material dispersed in silicon oil can form micron size nanocone “fibres” when a field of minimum 50 V/mm is applied. In order to form fibres within a reasonable time, fields of 400V/mm are used.
In Schwarz et al. Polymer 2002, 43, 3079 “Alternating electric field induced agglomeration of carbon black filled resins” it is reported how carbon black filled resins below zero-field percolation threshold can form electrically conductive networks when a field of 400 V/cm is applied between copper electrodes dipped into the resin.
US 20090038832 describes a method for forming an electrical path having a desired resistance from carbon nanotubes dispersed in a curable polymer matrix. Electrodes are placed in contact with the dispersion and electrical energy is applied to the carbon nanotubes until the desired electrical resistance is reached. A pure semi-conducting connection can be achieved by burning away metallic nanotubes that may be part of the carbon nanotube mixture, by applying a current after the deposition. The polymer matrix is cured in order to fix the device.
A disadvantage with prior art is that carbon nanotubes are very expensive and difficult to produce on an industrial scale. A dispersion of nanotubes is difficult to store and require specific manufacturing steps like homogenation or sonication prior to application of the dispersion to the substrate. The process of making holes in the polymer matrix to be filled by conductive materials as described in U.S. Pat. No. 4,101,689A is also complicated.
There is, therefore, a need for a more cost effective manufacturing method giving devices, films and adhesives with uniform conductivity and improved mechanical and optical properties.